This invention relates to high-resolution microscopy and, more particularly, to a scanning electron microscope (SEM) utilized for imaging and measuring purposes.
It is well known that SEMs are employed in a variety of scientific and commercial applications to perform a number of tasks of practical importance. By way of example, SEMs are widely used as a high-resolution monitoring tool in connection with the design, testing and fabrication of microminiature devices such as integrated circuits (ICs).
As the minimum feature size of IC devices has continued to decrease, it has become apparent that the accuracy, sensitivity, contrast and stability of conventional SEMs are often inadequate to meet the critical requirements associated with imaging and measuring features on such devices. Also, the necessity of being able to align IC features in the SEM apparatus is apparent. As feature sizes shrink, the importance of being able to perform this alignment in a simple and direct way with extremely high precision is becoming vital.
Heretofore, various expedients have been resorted to to try to improve the performance of conventional SEMs in certain critical applications such as those mentioned above. These expedients have, for example, included varying the operating parameters of the SEM, often in a trial and error way, to attempt to maximize some property of the signal obtained from the sample under study. More often than not, however, such approaches have at the same time resulted in degrading other important capabilities of the SEM.
In some cases, electronic processing of the signals obtained from the SEM can serve to enhance their suitability for a particular application. But such post-scanning processing is typically limited in its enhancement capabilities and often requires a considerable amount of expensive processing equipment.
With respect to the matter of aligning a sample in the SEM apparatus, this is often done manually by an operator based on visual inspection. This approch has obvious limitations when applied, for example, to sub-micro-size features on IC devices. Further, other alignment approaches based on sophisticated pattern recognition routines also have been found to have practical drawbacks.
Accordingly, efforts have continued by workers skilled in the art directed at trying to improve the aforementioned operating characteristics of SEMs for use in applications of the type mentioned above. It was recognized that these efforts, if successful, could significantly improve the performance of SEMs in general and in particular make them especially attractive for use in connection with the design, testing and manufacture of small-feature-size ICs.